This invention pertains generally to methods for improving the performance of detectors for gamma-ray and x-ray spectrometers and imaging systems. More particularly, this invention pertains to a method for treating the surface of CdZnTe detector crystals to reduce leakage currents and increase spectral resolution.
Many of the commonly used radiation detectors employ Si(Li) or Ge semiconductor materials and thus operate most effectively at cryogenic temperatures and under very clean vacuum conditions. The need to operate Si(Li) or Ge-based detectors under these rigorous conditions poses significant limitations on their use for applications where portability is desired.
The general requirements for room temperature operation of semiconducting materials as detectors for spectrometer applications are numerous and in some case contradictory. In particular, in this important that the chosen material exhibit a relatively large band gap energy such that thermal generation of charge carriers is minimized. Conversely, however, a small band gap energy is necessary such that a large number of electron-hole pairs is created for each absorbed quantum of ionizing radiation in order to maximize detector resolution. In addition, the material under consideration should have a relatively high average atomic number when used in gamma ray spectroscopy to increase the gamma ray interaction probability. Lastly, a high charge carrier mobility and long charge carrier lifetime are needed to ensure efficient charge carrier extraction and minimal effects from position dependent charge collection.
CdZnTe (CZT), and particularly Cd1xe2x88x92xZnxTe (where x is less than or equal 0.5), is a wide bandgap ternary II-VI compound semiconductor that, because of its unique electronic properties, is desirable for use in room temperature gamma-ray and x-ray radiation detection, spectroscopy, and medical imaging applications. However, the performance of gamma-ray and x-ray spectrometers which employ CZT detector crystals is often limited by surface leakage currents which act as a source of noise that reduces the ability of these spectrometers to spectrally resolve the unique radiological emissions from a wide variety of radioactive isotopes. Thus, in order to improve the spectral resolution capability of devices based on CZT crystals it is desirable to decrease surface leakage currents and the attendant detrimental noise effects.
It is known, in the art, that for a semiconductor crystal to function effectively as a good detector material (i.e., minimizing surface leakage currents, thereby maximizing energy resolution) the crystal surfaces must be properly xe2x80x9ctreated.xe2x80x9d Generally speaking, this means chemically etching of the surfaces to eliminate undesirable surface features. Currently, the generally accepted method for surface treatment of CZT crystals is to chemically etch the crystal surfaces in a solution of liquid bromine dissolved in methanol in order to provide a planar surface prior to attachment of electrical contacts. These solutions, or xe2x80x9cetchants,xe2x80x9d are used because they reliably produce surfaces on CZT crystals that are substantially planar and that have a low surface leakage current.
Applicant have also shown that it is possible to markedly reduce leakage current, and therefore noise discrimination, in these crystals by incorporating a passivating layer on the surfaces of the crystal. Such a system is described in co-pending U.S. patent application Ser. No. 09/118,691 wherein a silicon nitride layer is sputtered onto a CZT crystal surface as such a passivating layer.
However, there is a need to reduce the surface leakage current in CZT crystals still further in order to improve spectral resolution. What is required is a method for surface treatment of CZT crystal that will eliminate or reduce surface leakage currents to a level that is presently unattainable using prior art methods.
CdZnTe (CZT) crystals, particularly Cd1xe2x88x92xZnxTe (where x is less or equal 0.5) crystals and preferably Cd0.9Zn0.1Te crystals, are useful for fabrication of small, portable, room temperature radiation detectors. In a first embodiment of the present invention a method is taught for treating a surface or surfaces of CZT crystals that provides a coating on the crystal surface which will reduce surface leakage currents to a previously unattainable level and thereby provide for improved energy resolution in instruments incorporating CZT crystals processed by this invention. A two step process is disclosed, wherein the surface of a CZT crystal is etched the traditional bromine/methanol etch treatment (5 volume percent (v/o) bromine in methanol solution), and after attachment of electrical contacts the surface of the CZT crystal is passivated, preferably by treatment with a aqueous solution of ammonium fluoride and hydrogen peroxide.
A second embodiment of the present invention provides for a surface treatment of CZT crystals that reduces surface leakage currents and simultaneously provides a hard-coat over-layer which prevents performance decay over time due to exposure to moisture and other gases in the working environment, thereby providing for improved energy resolution and reliability. This second embodiment includes the process of the first embodiment followed with an encapsulation technique comprising a low-temperature sputter-deposited silicon nitride overlayer or the application of a polymer seal layer known as HumiSeal(copyright) (HumiSeal(copyright) is a registered trademark of the Chase Corporation, Woodside, N.Y.).
It is an object of this invention therefore to provide a method for producing a passivating dielectric layer on the surface of a CZT crystal.
It is another object of this invention to provide a method for producing a thick, dense, coherent dielectric layer on the surface of a CZT crystal.
Yet another object of this invention is to provide an enhanced oxidizing solution treatment for producing a thick, dense, coherent dielectric layer on the surface of a CZT crystal.
Still another object of this invention is to provide a method for passivating a CZT crystal surface by immersing said surface in an oxidizing solution comprising hydrogen peroxide and ammonium fluoride in water.
Yet another object of this invention is to provide a method for passivating a CZT crystal surface by immersing said surface in an oxidizing solution comprising 10 w/o hydrogen peroxide and 10 w/o ammonium fluoride in water.
Another object of this invention is to provide a method for reducing surface current leakage in CZT crystals by factor of about 5 below that achievable by conventional wet chemical methods.
Yet another object of this invention is to provide a CZT detector incorporating a CZT crystal having a thick, dense, coherent native oxide dielectric layer covering all exposed surfaces
Yet another object of this invention is to provide a CZT crystal having a dense, coherent oxide coating that consists essentially of cadmium oxide.
Another object of this invention is to provide a CZT crystal having an oxide coating which at least greater than about 250 xc3x85.
Still another object of this invention is to provide a high resistivity CZT crystal having a surface current leakage below about 0.01 nA.
A further object of this invention is the application of moisture barrier over the passivating layer such as either a low-temperature sputter-deposited overlayer or a moisture impermeable polymer layer.